1. Field of the Invention
The present invention relates to an engine speed calculating apparatus for computing the number of revolutions per minute at which an engine is turning.
2. Description of the Related Art
In a conventional arrangement for engine speed calculation, a time required for a crankshaft to revolve through a particular predetermined angle is measured and an engine speed is determined on the basis of measured result data of the time required for revolution through the predetermined angle. More specifically, a time required for revolution through a predetermined angle of 360 deg. is measured according to the generation timing of a crank pulse signal attained from a crankshaft position sensor, and then the reciprocal of the measured result value of the time required for 360 deg. of revolution is multiplied by 60, for example, to determine an engine speed. The engine speed thus determined is used as an important engine operation parameter in engine control operation such as fuel injection control (as disclosed in Japanese Unexamined Patent Publication No. Sho 61-277845 and No. Hei 9-264241, for example).
In a low engine speed range, however, a certain degree of pulsation occurs in revolutions of an engine even when a constant engine speed is to be provided, whereas such pulsation does not tend to occur in a high engine speed range. Therefore, in a calculation of a low engine speed using a time required for revolution through the same angle as in the high speed range, the result of the calculation represents a value of engine speed affected by pulsating revolutions, thus causing variations in engine speed values calculated in succession. Where such calculated result data of the engine speed affected by pulsating revolutions is used for engine control, there is a problem that satisfactory engine control is not performed.
This disadvantageous condition is particularly significant in an engine featuring unequal-interval ignition such as a V-type engine.
For instance, in an L-head four-cylinder engine featuring ordinary equal-interval ignition, each cylinder performs ignition (explosion) each time a crankshaft revolves through an angle of 180 deg. as shown in FIG. 1(a). In the sequence of first, third, second and fourth cylinders, ignition is performed repeatedly. More specifically, as shown in FIG. 1(b), each of expansion, exhaust, induction and compression strokes in each cylinder is performed synchronously at crank angle intervals of 180 deg. in a fashion that the first to four cylinders have mutually different kinds of strokes at each crank angle interval of 180 deg. in a crank angle cycle of 720 deg. Accordingly, during a period corresponding to each crank angle interval of 180 deg., each of the expansion, exhaust, induction and compression strokes is performed in any one of the four cylinders respectively, i.e., the same kind of stroke is not taken simultaneously in a plurality of the cylinders. The expansion stroke for accelerating the crankshaft with downward movement of a piston forced by combustion pressure and the compression stroke for compressing an intake air-fuel mixture to cause deceleration of the crankshaft take place simultaneously in crank angle intervals of 180 deg. Therefore, a relatively high degree of balance is maintained to equilibrate revolutions of the crankshaft, and engine speed calculation is not likely to be affected by pulsating revolutions except for variations among the cylinders.
By way of contrast, in a V-type four-cylinder engine featuring unequal-interval ignition, ignition (explosion) is performed repeatedly in the sequence of first, third, second and fourth cylinders as shown in FIG. 2(a). After ignition in the first cylinder at a crank angle of 0 deg., a crank angle interval of 180 deg. is taken until ignition in the third cylinder, a crank angle interval of 270 deg. is taken between ignition in the third cylinder and ignition in the second cylinder, a crank angle interval of 180 deg. is taken between ignition in the second cylinder and ignition in the fourth cylinder, and then a crank angle interval of 90 deg. is taken between ignition in the fourth cylinder and ignition in the first cylinder. More specifically, as shown in FIG. 2(b), although each of expansion, exhaust, induction end compression strokes in each cylinder is performed at crank angle intervals of 180 deg., each stroke transition point is different by 90 deg. between a pair of the first and third cylinders and a pair of the second and fourth cylinders. Therefore, in a cycle in which a crankshaft revolves two turns from a crank angle of 0 deg. to a crank angle of 720 deg., the number of ignitions during a one-turn period from a crank angle of 360 deg. to a crank angle of 720 deg. is larger than that during a one-turn period from a crank angle of 0 deg. to a crank angle of 360 deg. Since there is a crank angle interval of 270 deg. between a point in time of ignition in the third cylinder and that in the second cylinder, the crankshaft tends to be decelerated immediately before ignition in the second cylinder. On the contrary, since there is a relatively short interval of 90 deg. between a point in time of ignition in the fourth cylinder and that in the first cylinder, the crankshaft tends to be accelerated immediately before and after ignition in the first cylinder. During a period from 0 deg. to 90 deg. in each cycle (period xe2x80x9cAxe2x80x9d indicated in FIG. 2(b)), the expansion stroke is duplicated in the first and fourth cylinders so that the crankshaft tends to be accelerated increasingly. Further, during a period from 360 deg. to 450 deg. (period xe2x80x9cBxe2x80x9d indicated in FIG. 2(b)), no expansion stroke takes place in any cylinder whereas the compression stroke is performed in the second cylinder, thereby causing a tendency of decelerating the crankshaft. Consequently, in the V-type four-cylinder engine or a similar engine featuring unequal-interval ignition, a degree of engine running pulsation in a one-turn period from a crank angle of 0 deg. to a crank angle of 360 deg. differs from that in the next one-turn period from a crank angle of 360 deg. to a crank angle of 720 deg. In a low engine speed range in particular, engine speed calculation is likely to be affected by pulsating revolutions.
It is therefore an object of the present invention to obviate the above mentioned disadvantage by providing an engine speed calculating apparatus which is capable of performing engine speed calculation without being affected by pulsating revolutions in a low engine speed range.
In accomplishing this object of the present invention and according to one aspect thereof, there is provided an engine speed calculating apparatus. The apparatus is for measuring a time required for a crankshaft of an engine to revolve through a predetermined angle upon each completion of revolution through the predetermined angle end calculating an engine speed on the basis of measured result data of the time required for revolution through the predetermined angle. The apparatus includes determining means for determining whether the preceding calculated engine speed is lower than a predetermined engine speed; and calculating means for performing engine speed calculation on the basis of a time required for the crankshaft to revolve through an angle larger than the predetermined angle when the preceding calculated engine speed is determined to be lower than the predetermined engine speed.
In the above aspect of the present invention, an engine speed in a low speed range is calculated on the basis of a time required for the crankshaft to revolve through an angle larger than the predetermined angle. Since the engine speed is thereby averaged to a degree larger than that in a high speed range, engine speed calculation can be performed without being affected by pulsating revolutions. Further, in a high speed range where a degree of engine running pulsation is relatively low, an engine speed is calculated on the basis of a time required for the crankshaft to revolve through the predetermined angle. Therefore, it is possible to perform accurate engine speed calculation while following up variations in actual engine speed.
Furthermore, according to another aspect of the present invention, in engine speed calculation teased on a time required for the crankshaft to revolve through an angle larger than the predetermined angle, a multiple of the predetermined angle is used as the angle larger than the predetermined angle. Thus, a time required for revolution through the predetermined angle is measured constantly. In the high speed range, each period of time required for revolution through the predetermined angle is used directly to calculate an engine speed. In the low speed range, the sum of a plurality of consecutive periods, each representing a time required for revolution through the predetermined angle, is used to calculate an engine speed.
This arrangement facilitates the processing of engine speed calculation.